1. Field of the Invention
The invention relates to an apparatus for the vaporization of liquid precursors and the controlled delivery of those precursors to form films on suitable substrates. More specifically, this invention concerns an apparatus for the deposition of a high dielectric constant film on a silicon wafer to make integrated circuits useful in the manufacture of advanced dynamic random access memory modules and other semiconductor devices.
2. Description of the Related Art
As the dimensions of the transistors continue to decrease, the continued use of silicon dioxide as a dielectric gate material is problematic. The fundamental problem is the need to keep the capacitance of the gate high while the area of the gate is shrinking faster than the thickness of the gate dielectric stack. The capacitance C of the gate is given by C=k∈oA/d where A is the area of the gate, d is the thickness of the dielectric stack, k is the dielectric constant, and ∈o is the permittivity of free space. To ensure higher gate oxide capacitance, the silicon dioxide layer thickness has been decreased to less than 2 nanometers and future generations may require a further reduction below 1.0 nanometer. Since the dominant transport mechanism for silicon dioxide (SiO2) films less than approximately 3 nanometers thick is by direct tunneling of electrons or holes, the leakage current density increases exponentially with decreasing thickness. A typical leakage current density for 1.5 nanometers thick SiO2 at 1 V is about 1 A/cm2. But, as the SiO2 thickness approaches 1 nanometer, the leakage-current density approaches an unacceptable 100 A/cm2 at the same operating voltage.
Consequently, there is a need for an alternative gate dielectric material that can be used in a large enough physical thickness to reduce current leakage density and still provide a high gate capacitance. In order to achieve this, the alternative gate dielectric material must have a dielectric constant that is higher than that of silicon dioxide. Typically, the thickness of such an alternative dielectric material layer is expressed in terms of the equivalent oxide thickness (EOT). Thus, the equivalent oxide thickness (EOT) of an alternative dielectric layer in a particular capacitor is the thickness that the alternative dielectric layer would have if its dielectric constant were that of silicon dioxide.
Another consideration in selecting an alternative dielectric material is the mobility of electrons in the transistor channel. The material selected for the dielectric film effects the mobility of the carriers in the transistor channel, thereby affecting overall transistor performance. Thus, it is desirable to find an alternative dielectric material for which the mobility of carriers in the transistor channel is equivalent to or higher than that for silicon dioxide gate dielectric films. For future generation transistors, a peak mobility of 400 cm2/V·s or greater is desirable.
This drive toward smaller transistors is driven by the desire for more integrated circuits (ICs) on a semiconductor die. Manufacturers are interested in replacing today's 64 megabit DRAM with memory devices in the range of 256 megabit, 1 gigabit, and higher. This need for more ICs on the same or smaller substrate footprint makes it necessary to replace conventional dielectric films, such as SiO2, with dielectric films having higher dielectric constants (“High k” films).
High k films are desirable because their higher dielectric constants mean they provide higher capacitance that enables closer spacing of devices without electrical interference. Such closer spacing can increase transistor density. In addition, capacitor size can be reduced because capacitors containing high dielectric constant materials, such as tantalum oxide (Ta2O5), usually have much larger capacitance densities than standard SiO2—Si3N4—SiO2 stack capacitors. In fact, tantalum oxide has a relative dielectric constant more than six times that of SiO2. Thus, High k materials such as tantalum oxide are becoming the materials of choice in IC fabrication.
One common method of forming a tantalum oxide film is to vaporize a liquid tantalum precursor and then deliver the tantalum vapor to a deposition chamber. FIG. 1, which is a graph of Vapor Pressure (Torr) vs. Temperature (□ C) of various compositions, graphically illustrates the large variation among the vapor pressures of tantalum precursors and other representative prior-art precursors for other semiconductor related processes. For example, at 100□ C and 1 atm TAT-DMAE has about 0.3 Torr vapor pressure while TAETO has about 0.03 Torr vapor pressure. The vapor pressures for tantalum precursors are remarkably lower than those of precursors typically used in prior art vapor delivery systems. Again referring to FIG. 1, at 100 C and 1 atm, TEOS (Tetra Ethyl Ortho Silicate), which is commonly used in chemical vapor deposition processes to form SiO2 films and is supplied by several prior art vapor delivery systems, has a vapor pressure of almost 100 Torr. As a result of the vast difference in vapor pressure illustrated by TAETO and TEOS, prior art vapor delivery systems do not encounter and do not provide solutions to many of the challenges resulting from the use of very low vapor pressure precursors such as TAETO and TAT-DMAE.
Prior art vapor delivery systems commonly use an integrated liquid flow controller and vaporizer without a positive liquid shut-off valve. Such a configuration, when used with low vapor pressure tantalum precursors, can lead to problems stabilizing the tantalum vapor output and difficulty achieving the constant, repeatable tantalum vapor output desired in semiconductor device fabrication. Prior art delivery systems for TEOS and other relatively high vapor pressure materials allow for the flow controller and vaporizer to be separated by a considerable distance or attach no significance to the distance between vaporizer and liquid flow meter. Positioning the vaporizer and flow meter according to prior art systems fails to adequately control precursor vapor in the case of low vapor pressure precursors.
Previous delivery systems also have cleaning systems that are intended for use with higher vapor pressure precursors whose residuals can be adequately removed (“purged”) by applying low pressure or “pumping-down” the lines while flowing a gas like nitrogen that is inert, relative to these materials. Purging techniques such as these fail with tantalum systems because the residual tantalum precursor has such a low vapor pressure that to remove it a system must introduce a solvent, such as isopropyl alcohol, ethanol, hexane, or methanol, into both the vaporization system and supply lines.
Previous vapor delivery systems avoided precursor vapor condensation by heating the delivery lines. These heating systems usually resorted to a flexible resistive heater that was wrapped around and held in direct contact with the line and then insulated. Since such systems typically operated with precursor materials having a wide temperature range within which the precursor remained vaporous, they did not need to sample the temperature of the heated line in as many locations. Typically, a single thermocouple would be used to represent the temperature of piping sections as long as four to six feet. Unfortunately, since the object of these large scale temperature control systems is to heat and monitor an average temperature of a large section of piping, these systems lack the ability to specifically control a single, smaller section of the vapor piping. An additional detriment is that these systems generally have very low efficiency when higher line temperatures are desired.
Vaporized tantalum delivery systems need to maintain the tantalum vapor above the vaporization temperature but below the decomposition temperature for a given tantalum precursor. Thus, once formed, the vaporous tantalum must be maintained at elevated temperatures between about 130° C. and 190° C. for TAT-DMAE and between about 150° C. and 220° C. for TAETO. Because of the relatively high temperatures needed and the narrow temperature band available to low vapor pressure precursors such as TAT-DMAE and TAETO, tantalum and other low vapor pressure liquid delivery systems would benefit from vapor delivery line temperature controls and methods that can achieve and efficiently provide the higher temperatures and greater temperature control needed for tantalum vapor delivery. Additionally, more precise temperature controls are needed since the usable temperature range of vaporized low pressure liquids is smaller than the usable range of prior art liquids. Because higher temperature vapor delivery is needed, tantalum delivery systems would benefit from designs that minimize the length of heated vapor delivery lines. Minimizing the length of lines requiring heating not only reduces the overall system complexity but also decreases the footprint or overall size of the system.
Current methods of tantalum oxide deposition use reaction rate limited chemical vapor deposition techniques. In reaction rate limited deposition processes, the deposition rate achieved is largely influenced by the temperature of the reaction environment. Existing chemical vapor deposition reactors do not sufficiently address the thermal losses from the substrate onto which the tantalum film is to be formed and the internal chamber components such as the gas distribution showerhead. Such thermal losses result in a non-uniform thickness of deposited tantalum and this non-uniformity is one barrier to having commercially viable tantalum oxide film formation techniques. Also, a commercially viable tantalum deposition requires a viable, in-situ cleaning process that can remove tantalum deposition formed on internal chamber components without harm to these components.
Thus, there is a need for a deposition apparatus that can deliver vaporized, measured High k precursors, such as tantalum, hafnium, or zirconium precursors, that have been adequately mixed with process gases to a reaction chamber that provides a controlled deposition environment that overcomes the shortcomings of the previous systems.